When an upper nozzle formed with a bore for allowing molten steel to flow therethrough is used in a state in which it is fitted into a well block of a tundish, inclusions, such as alumina cluster, and metals are apt to adhere to a wall surface defining the bore. As a result, a flow passage in the bore is narrowed. In this case, it is necessary to remove the adhered substances by cleaning the inner hole using a bar or the like, or using an oxygen lance, thereby causing a hindrance to casting operation. In some cases, the bore is completely clogged by the adhered substances, thereby falling into a situation where it becomes impossible to continue the casting operation. Therefore, various techniques for preventing the occurrence of such adhesion have heretofore been invented and proposed.
For example, the following Patent Document 1 proposes an upper nozzle formed with a bore having a shape capable of creating a less energy loss or smooth (constant) molten steel flow to suppress the occurrence of the adhesion.
The following Patent Document 2 proposes a continuous casting insert nozzle (upper nozzle) formed with a bore for allowing molten steel to flow therethrough, wherein the insert nozzle comprises a porous refractory member (gas-permeable refractory member) defining the bore, thereby fulfilling a function of injecting inert gas into the bore.